DIY LED indicator for DIY 9V audio amplifier ? - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

Jolly 7 · 5th May 2018, 1:48 am

Most useful, thanks so much. I substituted the old resistor with a 33K one and the white LED is not so bright anymore, but more than adequate for my purposes. As per my calculations, it is now drawing only 0.3 mA which is remarkable.

In relation to the amplifier itself, it is based on a TDA2822 chip and is driving a large vintage 3 ohm electrodynamic speaker quite well, without need for any transformer etc.

mhennessy · 5th May 2018, 10:02 am

I've just searched and found a complete module using that chip for just £1.27 with free postage. It even includes a volume control (and knob). I never cease to be amazed at what you can get for next to nothing... The TDA2822 is another chip that went obsolete many years ago yet is still being supplied somehow. Certainly a worthwhile upgrade on the LM386 - not least because it's stereo

Glad it's sorted now

Argus25 · 6th May 2018, 12:31 am

As I mentioned on post 22, for battery powered apparatus, it is good if possible to get more value/function from the LED. Just wired with a resistor, it doesn't tell you much about the state of the battery.

One simple and easy trick for gear running from 9V, with LED's dropping 2 to 3V is to put a 3 to 4.7V zener diode in series with the LED & its resistor. It means you use a smaller value series resistor , but you can still have a good brightness LED. This way, as the battery voltage crashes below 5 to 6V, or thereabouts, the LED tends to fairly suddenly go out, letting you know the battery needs changing/charging. And adding a single zener diode is cheap.

Jolly 7 · 6th May 2018, 12:46 am

I might have a suitable Zener lying around and will use it as advised . Thanks for the very useful tips. Hope to post a pic of the amplifier in situ once all done. At the moment I am fitting a 9V DC socket to the cabinet for use with a DC adaptor as and when needed.

julie_m · 6th May 2018, 2:06 am

The Zener trick is a good one.

I did think of using an LM2903 to light the LED only under no-signal conditions (since you should be able to hear the amplifier working anyway, when there is something playing through it) but this probably would also end up using at least as much power as it saved.

If you wanted to be really frugal, you could add a small push-to-make switch to use the LED as a "battery test" light; with perhaps an extra, standard diode cunningly wired so that the LED will always light up when the unit is powered via the DC jack.

5th May 2018, 1:48 am	#41
Jolly 7 Octode Join Date: Feb 2018 Location: Southampton, Hampshire, UK. Posts: 1,061	Re: DIY LED indicator for DIY 9V audio amplifier ? Most useful, thanks so much. I substituted the old resistor with a 33K one and the white LED is not so bright anymore, but more than adequate for my purposes. As per my calculations, it is now drawing only 0.3 mA which is remarkable. In relation to the amplifier itself, it is based on a TDA2822 chip and is driving a large vintage 3 ohm electrodynamic speaker quite well, without need for any transformer etc.

5th May 2018, 10:02 am	#42
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: DIY LED indicator for DIY 9V audio amplifier ? I've just searched and found a complete module using that chip for just £1.27 with free postage. It even includes a volume control (and knob). I never cease to be amazed at what you can get for next to nothing... The TDA2822 is another chip that went obsolete many years ago yet is still being supplied somehow. Certainly a worthwhile upgrade on the LM386 - not least because it's stereo Glad it's sorted now

6th May 2018, 12:31 am	#43
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: DIY LED indicator for DIY 9V audio amplifier ? As I mentioned on post 22, for battery powered apparatus, it is good if possible to get more value/function from the LED. Just wired with a resistor, it doesn't tell you much about the state of the battery. One simple and easy trick for gear running from 9V, with LED's dropping 2 to 3V is to put a 3 to 4.7V zener diode in series with the LED & its resistor. It means you use a smaller value series resistor , but you can still have a good brightness LED. This way, as the battery voltage crashes below 5 to 6V, or thereabouts, the LED tends to fairly suddenly go out, letting you know the battery needs changing/charging. And adding a single zener diode is cheap.

6th May 2018, 12:46 am	#44
Jolly 7 Octode Join Date: Feb 2018 Location: Southampton, Hampshire, UK. Posts: 1,061	Re: DIY LED indicator for DIY 9V audio amplifier ? I might have a suitable Zener lying around and will use it as advised . Thanks for the very useful tips. Hope to post a pic of the amplifier in situ once all done. At the moment I am fitting a 9V DC socket to the cabinet for use with a DC adaptor as and when needed.

6th May 2018, 2:06 am	#45
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: DIY LED indicator for DIY 9V audio amplifier ? The Zener trick is a good one. I did think of using an LM2903 to light the LED only under no-signal conditions (since you should be able to hear the amplifier working anyway, when there is something playing through it) but this probably would also end up using at least as much power as it saved. If you wanted to be really frugal, you could add a small push-to-make switch to use the LED as a "battery test" light; with perhaps an extra, standard diode cunningly wired so that the LED will always light up when the unit is powered via the DC jack. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.